I think that the LegacyURL field is something for a specific site that accidentally got put into the main codebase! :-P
Are the old URLs in any consistent format? Your two basic solutions are setting up RedirectorPage pages with the old URLs, if the are of the form http://www.mysite.com/(url)/
If they are mostly in a couple of directories, you could set up director rules that point to a custom controller that does the redirection.
For example, if you have a bunch URLs of the form http://www.mysite.com/products/(XXX).html,
you could set up a director rule
'products/$ProductURL' => 'ProductRedirector'
and then make a class
class ProductRedirector extends Controller {
function index() {
// do something with $this->urlParams['ProductURL'];
}
}